Placing grass in only certain places
Assuming that you are using a particle system for your grass, you can use weight paint mode to define where your grass should be. Select the mesh and enter weight paint mode. Weights vary from 1 (red) to 0 (blue). Paint red where you want the grass to be most dense and blue where you don't want there to be any grass. Then, in the particle settings, go to Vertex Groups and where it says density, select your vertex group.